I'm using and reccommend the NAVMAN N60i. It's portable, has it's own battery so can be used in or out of a vehicle, plus it also has a built in digital camera. So instead of entering in a location you want to save, you just take a picture of it and next time you want to go there you just select that picture from your picture library. It's a lot easier than entering in an address. It also has a load of other neat features.
Cheers